Torque motors working as magnetic springs can be used for applications requiring rotary motion to develop torque with limited angle of rotation. Magnetic springs accomplish this through attractive and repulsive electromagnetic torques of the rotor with respect to the stator. In some particular cases, an additional rotation of the stator is required. The additional degree of mechanical freedom should not affect the level of the electromagnetic torque developed in both directions of rotation for completed revolutions.
Our U.S. Pat. No. 4,871,040 issued Oct. 3, 1989, to Zuraski et al. entitled "Electromagnetic Control Apparatus for Varying the Driver Steering Effort of a Hydraulic Power Steering System", is an application of one version of an actuator utilizing the principles of this invention. In particular, it has two degrees of freedom for conjoint rotation of the rotor and stator, as well as magnetically variable torque developed between the rotor and stator as a function of relative displacement and energizing current. The mechanism is used in conjunction with a torsion bar to center two portions of a power steering valve. By varying the current to the electromagnetic coil, the net centering force on the valve is varied and the steering effort is therefore controlled.
The magnetic springs can be used, of course, with a one degree of freedom configuration where the stator is stationary and only the rotor moves. Further, since the direction of torque is dependent on the direction of energizing current, the actuator can be used effectively to develop centering torque or anti-centering torque in either direction from a center position, depending on the direction of the energizing current. This leads to application as a three position actuator having stable states when centered or when driven to stops on either side of center. The actuator can also be configured to seek an off-center position as a function of current where the actuator movement is restrained by an external spring force, such that for a given current, there is a specific position where the torque balances the spring force.
The electromagnetic mechanism of this invention includes a permanent magnetic circuit and an electromagnetic circuit. The permanent magnetic circuit comprises a pair of relatively rotatable elements, one of which is toothed to conduct magnetic flux, and one of which includes permanent magnets for establishing a permanent magnet coupling. The electromagnetic circuit comprises an energizing coil and an external magnetic circuit, and also shares the toothed element.
In one illustrated embodiment, the toothed element is defined by a pair of axially spaced magnetic pole pieces, and the permanent magnet element is a rotor defined by a disk element disposed between the magnetic pole pieces. The disk element is supported for rotation with an output shaft, and the pole pieces are supported for rotation with another shaft. The disk element is axially magnetized to define an even number N of radially extending, alternating magnetic polarity sectors. The pole pieces each have N/2 teeth extending toward the respective axial face of the disk element. The electromagnetic circuit comprises at least one annular exciting coil disposed about the rotary magnetic circuit and ferromagnetic pole elements positioned adjacent the magnetic pole pieces.
The above elements define two magnetic flux paths: a permanent magnet flux path which includes (neglecting leakage flux) only the rotary disk element and the pole pieces, and an electromagnetic flux path which includes the coil, pole elements, pole pieces and the disk element. The pole pieces and the disk element are oriented such that for a first polarity of energizing current (1) when the assembly is in the centered position, both flux paths are magnetically balanced, and (2) when there is relative angular displacement of the input and output shafts from center, the flux in the two paths develop in-phase centering forces which tend to restore the assembly to the centered position.
The force due to the electromagnetic flux path is variable over a wide range depending on the magnitude and direction of current supplied to the coil, such that for a second polarity of energizing current the permanent magnet centering force may be overcome and (1) when the assembly is in the centered position it is in a state of unstable equilibrium, and (2) when there is relative angular displacement of the input and output shafts from center, the overriding force due to the electromagnetic flux path tends to drive the assembly away from the centered position.
In another embodiment, the pole pieces are stationary instead of being fixed to a rotatable shaft, thus offering one degree of freedom instead of two. The principle of operation is the same in either case.